1. Field of the Invention
The present invention relates to a sheet processing apparatus performing a sheet binding process and an image forming apparatus in which this sheet processing device is installed. The invention relates in particular to a sheet processing apparatus which can solve the problems of paper jamming and alignment defects for a binding operation and an image forming apparatus in which this sheet processing apparatus is installed.
2. Description of the Related Art
In the past, a sheet processing apparatus has been provided which performs aligning, binding by stapling, bending, and spine attachment (these processes together being sometimes called book making) for a plurality of sheets (a sheet bundle) on which images have been formed. In the past, there were many of the large console type sheet processing apparatus (a stationary unit that is separate from the image forming apparatus), but in recent years there has been demand for small sheet processing apparatus which is detachably attachable to the upper portion of a small or medium size image forming apparatus.
One example of this type of small sheet processing apparatus is that described in Japanese Patent Application Laid-Open No. 2003-246546 (Patent Document 1). In this apparatus, the space for movement of the stapler is eliminated by fixing the stapler, which is an example of a binding means, on the intermediate stacking portion, and the sheets are stacked and aligned at the opening of the stapler or in other words, between the driver (the metal plate that punches out the staples) and the clincher (the bending plate) and the end of the bundle of sheets is thereby stapled.
In this type of sheet processing apparatus, when the sheets are stacked in the intermediate stacking portion, because the sheets pass the opening of the stapler, the sheet aligning distance is short and the aligning time is also short. Also, there is no need to lead the sheet bundle to be stapled to the opening of the stapler after the sheets are aligned, and thus the situation does not occur where a curled sheet end contacts the stapler and causes stapling error. Furthermore, because a mechanism for leading the sheet bundle to the stapler becomes unnecessary, there is the advantage that the apparatus may become more compact and the cost may be reduced.
In the sheet processing apparatus having the above-described structure, in order to make it possible to process a large number of sheets, the height of the sheet bundle must be increased and stacking must be possible even when there is curling. Thus the height of aligning member must be increased and the possible stacking height of the intermediate stacking portion must also be increased. As a result, the nip position of the roller pair that conveys sheets to the intermediate stacking portion from the sheet stacking surface of the intermediate stacking portion must also be made high.
Meanwhile, as described above, the sheet bundle must be conveyed to the opening of the stapler (between the driver and the clincher) and aligned. However, while the possible stacking height of the intermediate stacking portion takes into consideration curling of the sheet and is set such that there is some leeway with the height, the height of the opening of the stapler is set with the thickness of the sheet bundle after being bound in accordance with the binding process capability of the stapler (the number of sheets that can be bound) as a reference. If the opening of the stapler is set with extra height, the stroke of the driver for binding processing will increase and corresponding strength is required and thus the apparatus becomes large. In addition, the binding processing time is increased because the stroke of the driver from the home position to the binding position is increased. Thus in the case where a stapler which is a binder that is small in size and has high processing efficiency is used, there is a limitation in that the opening cannot be placed too high. Consequently, there is need to increase the possible stacking height of the intermediate stacking portion and at the same time, provide a guide means for guiding the sheet to the opening of binder that has a limited height.
Because the stapler is fixed at one side of the intermediate stacking portion, the guiding means is disposed at the same side (in the vicinity of the stapler) and it guides the sheet at position lower than that of the other conveyance surfaces. As a result, the conveyance resistance is larger in the vicinity of the stapler than at the other conveyance surface. When the conveyance resistance of the guide means increases and the conveyance speed is reduced in this manner, lack of uniformity in the conveyance speed in the both sides of the width direction of the sheet which intersects the conveyance direction occurs and this results in slanting which sometimes causes paper jams. Furthermore, because the sheets are sometimes stacked slant at the time of stacking in the intermediate stacking portion and the aligning means does not manage to completely align the sheets, there is the problem that binding processing may be done without sufficient alignment. Problem of this kind is especially apparent with a thick, rigid sheet.